In drawing and spinning systems wherein sliver or roving is drawn, spun, doubled or twisted while being longitudinally stretched it is standard procedure to operate at extremely high speeds. Frequently the linear advance speed is well above 350 m/min, normally well above 500 m/min. This advance speed is established by one or more pairs of pinch rollers that engage all the strands pulled from their supplies at a distance downstream from all of these supplies, a distance frequently as small as 1 m.
If one of the strands breaks or one of the supplies runs out, it is essential to be able to stop the advance before the insufficiently plied portion of the strand reaches the advance rollers.
It is therefore standard procedure to use motors which incorporate heavy-duty mechanical brakes. When such a motor is de-energized the brake automatically acts and arrests the drive element contacting the yarn very rapidly.
Such a system has the considerable disadvantage that, due to its mechanical operation, it is subject to considerable wear and must be serviced frequently. The brake shoes must be changed occasionally and the brake must be adjusted for proper operation at frequent intervals. Even so when the brake becomes relatively worn it is frequently impossible for it to stop the yarn that is advancing within a short enough time to prevent the unplied yarn from entering the advance rollers.
It has also been suggested to drive the advance element for the filament by means of an asynchronous motor. Such a motor is normally powered by alternating current, but can be brought to a relatively rapid halt by powering it with direct current. This is problematic, however, since immediately after stopping of the multiphase alternating current feeding the motor it is necessary to wait a few seconds for the magnetic field to collapse. If this time is not waited before connecting up the power supply for direct current, back voltage can damage the diodes or thyristors used as rectifiers in the arrangement. Since several tenths of seconds must therefore be lost before the field collapses, such an arrangement normally does not act rapidly enough for high-speed systems.